Differential receivers have been provided to protect a pair of differential inputs from interference and noise degradation that occur when the differential inputs are transmitted over a transmission line. See, for example, U.S. Pat. Nos. 6,288,577, 6,650,149 and 6,879,198.
Typical differential receivers may be able to tolerate certain types of fault and degradation in the differential inputs. For example, they may be able to tolerate “stuck at” signal faults, which occur when the differential inputs remain at their logical levels and don't switch to subsequent logical levels. Typical differential receivers may not, however, be able to tolerate other types of signal faults such as “stuck at” faults with only one of the differential inputs stuck at a certain logic level, short and open faults as well as no signal faults. These faults generally cause both differential inputs to have equal or no differential voltage, thereby leading to an unpredictable output signal and behavior, including oscillations at the differential outputs.
For example, a typical prior-art differential receiver is shown in FIG. 1. Differential receiver 100 receives differential inputs 105 and 110 at opposite logical levels. Each differential input is coupled to a transistor, with differential input 105 coupled to the base of transistor 115 (“Q1”) and differential input 110 coupled to the base of transistor 120 (“Q2”). Current source 125 (“I1”) is coupled to the emitters of transistors 115 and 120, and resistors 130 (“R1”) and 135 (“R2”) are in turn coupled to the collectors of transistors 115 and 120, respectively. The collectors of transistors 115 and 120 are also coupled to the bases of transistors 140 (“Q3”) and 145 (“Q4”), respectively. The emitters of transistors 140 and 145 are respectively coupled to current sources 150 (“I2a”) and 155 (“I2b”). The emitters of transistors 140 and 145 are in turn coupled to output amplifier 160, which generates differential outputs 165 and 170.
Differential receiver 100 operates as follows. When differential input 105 is at a logical level opposite than that of differential input 110 with sufficient voltage difference, the differential outputs 165 and 170 deliver full swing output. For example, when differential input 105 is high and differential input 110 is low, transistor 115 is turned ON and transistor 120 is turned OFF. The collector current of transistor 115 corresponding to the current in current source 125 flows through resistor 130 resulting in a voltage drop across resistor 130, which will provide a differential voltage at the bases of transistors 140 and 145. Transistors 140 and 145 are always ON and form voltage followers, that is, differential signals 180 and 185 “follow” differential signals at the bases of transistors 140 and 145 respectively. This way, differential receiver 100 delivers a full output swing when differential inputs 105 and 110 are behaving normally. However, in the case when differential inputs 105 and 110 are stuck at the same voltage level due to some “stuck at” fault conditions, that is, the base terminals of transistors 115 and 120 have the same voltage, both transistors 115 and 120 will be turned ON. As a result, differential outputs 165 and 170 may be in an undetermined state or even oscillating.
To prevent this unpredictable behavior at the output, prior art attempts have been made to include a Schmitt-Trigger in a differential receiver. A Schmitt-Trigger, as generally used herein, may act as a memory to retain the value of the differential inputs when they are at an undetermined state, e.g., between a high and low logic levels, until they change sufficiently to “trigger” a change in state. A Schmitt-Trigger has been used, for example in the prior-art differential receiver illustrated in FIG. 2. Differential receiver 200 receives differential inputs 205 and 210 and generates differential outputs 215 and 220. In addition to having the same configuration of differential receiver 100 shown in FIG. 1, that is, in addition to having transistors 115 and 120, current source 125, resistors 130 and 135, transistors 140 and 145, current sources 150 and 155, and output amplifier 160, differential receiver 200 has Schmitt-Trigger 225 formed by transistors 230 (“Q5”) and 235 (“Q6”) and current source 240 (“I1b”). The bases of transistors 230 and 235 are connected to the collectors of transistors 115 and 120, respectively, and the collectors of transistors 230 and 235 are in turn respectively connected to the collectors of transistors 120 and 115, which are connected to the bases of transistors 145 and 140. Current source 240 is connected to the emitters of transistors 230 and 235.
Differential receiver 200 operates as follows. When differential input 205 is at a logical level opposite than that of differential input 210 with sufficient voltage difference, the differential outputs 215 and 220 deliver full swing output. For example, when differential input 205 is high and differential input 210 is low, transistor 115 is turned ON and transistor 120 is turned OFF. Current sources 125 and 240 and resistors 130 and 135 are configured such that transistor 235 is turned ON and transistor 230 is turned OFF.
The amount of hysteresis introduced by Schmitt-Trigger 225 can be programmed by current source 240, which is designed to ensure that one of transistors 230 and 235 is turned ON while the other is OFF when there is no voltage difference between inputs 205 and 210. This hysteresis would also reinforce the state of differential inputs 205 and 210 when there is sufficient voltage difference between inputs 205 and 210. Assuming right before a fault is developed, input 205 is high and input 210 is low, transistor 115 is turned ON resulting in a low voltage at the collector of transistor 115. When differential inputs 205 and 210 are stuck at the same voltage level for example, transistor 230 may try to turn itself ON from its OFF state, but it will be prevented from doing so because the collector of transistor 115 will be at a lower voltage than the collector of transistor 120.
This lower voltage will prevent current from current source 240 to flow to the collector of transistor 230. As a result, differential outputs 215 and 220 will retain their value and won't change to a different logic state until differential inputs 205 and 210 are at logic levels sufficient for turning transistor 230 ON, that is, at voltage levels sufficient for triggering a change in the logic states of differential outputs 215 and 220. Schmitt-Trigger 225 therefore prevents differential outputs 215 and 220 from oscillating or having an unpredictable behavior. When differential inputs 205 and 210 are behaving unexpectedly, differential outputs 215 and 220 may still behave appropriately and retain their logic values.
Although Schmitt-Trigger 225 prevents differential outputs 215 and 220 from behaving unpredictably, because it is in the data path between transistors 115 and 120 and transistors 140 and 145, it affects the bandwidth of the differential receiver 200 and causes waveform distortion. There is, therefore, a need to implement a fail-safe differential receiver that tolerates unpredictable behavior at the differential inputs without affecting its bandwidth or distorting the waveforms at the differential outputs.